The production of high resolution patterned images used in the fabrication of miniaturized circuits is important in modern technology. These circuits are useful in logic circuits, memory devices, etc., which, in turn, form the components of devices such as calculators, computers, etc. The production of the patterned images generally involves coating a wafer with a radiation curable material, commonly called a resist, and exposing selected portions of the resist to radiation which initiates reactions within the exposed portion of the resist and causes the exposed portion to become either more or less easily removed, i.e., more or less soluble, with respect to the unexposed portion when the resist is subjected to an appropriate developer. The circuit and device fabrication sequence thus begins with the resists being coated onto wafers composed of materials, e.g., silicon, used in the manufacture of large scale integration devices, e.g., random access memories, etc. The wafer thus serves as a substrate for the resist. After portions of the resist are exposed to radiation and the more soluble portions removed by development, the now bared substrate portions are modified by, e.g., removal of substrate material or deposition of new material on or into the substrate. The production of many types of devices, for example, semiconductor integrated circuits, requires that the described processing sequence be repeated several times, i.e., the remaining resist is stripped off after substrate modification and a new layer of resist is put down and exposed, etc. The patterned resist may itself also act as a dry processing mask to permit replication in an underlying and relatively thick second resist layer which accommodates substrate surface roughness and permits presentation of a smooth surface to the incident radiation.
A successful lithographic system, i.e., a system which produces high resolution patterned images, requires the availability of a resist which has several desirable properties including both good sensitivity to the incident radiation and good adhesion to the substrate. Furthermore, the resist should exhibit good reproducibility, both between batches of resist material and from coated wafer to coated wafer within a batch of resist material of these properties.
Several classes of materials have been considered as candidates for use as resists. Inorganic resists, such as those containing silver halides, have received some attention from those engaged in developing resists, but most of the effort directed to resist development has been devoted to organic resists. Recently, however, the effort directed toward the development of inorganic resists has greatly increased because of advantages theoretically offered by this class of resists. One such advantage of inorganic resists, as compared to organic resists, is a generally high absorption cross-section for most commonly used electromagnetic radiation. This results in essentially total radiation absorption within the usual resist layer thickness and thus avoids any depth dependence of the exposure caused by standing waves.
One example of an inorganic resist is described in Applied Physics Letters, 31, pp. 161-163, Aug. 1, 1977. The inorganic resist described is, before exposure, in the form of a thin silver layer supported by a selenium-germanium radiation absorbing layer that is disposed on a substrate. The resist operation depends on the photoinduced silver migration into the exposed portions of the selenium-germanium, i.e., chalcogenide layer to reduce the solubility of the exposed, i.e., silver containing, portions of the chalcogenide layer in an alkaline developer. Another inorganic resist is described in Journal of Vacuum Science and Technology, 16, pp. 1977-1979, November/December, 1979. This article described a resist system based on the photoinduced migration of silver into any member of a family of glassy chalcogenide materials. The system regularly attains submicron resulution. The silver is introduced into the resist not from an elemental form as in the previously mentioned Applied Physics Letters but from either a combined or complexed form which is chosen to interact with one or more components of the glassy layer to yield a layer of a silver compound disposed on the underlying glassy layer. The silver compound so formed then serves as a migration source of silver ions when the resist is exposed to radiation. It is hypothesized that the photoinduced migration proceeds through a mechanism that involves electron-hole pair generation by absorbed photons with the holes combining with silver. The resulting migration is thus truly the migration of silver cations.
While a number of methods and silver containing compositions may be utilized to introduce the silver containing layer onto the glassy chalcogenide layer, it was found convenient to form the silver containing layer by dipping the chalcogenide glass coated wafer in an aqueous solution, commonly called a sensitizing bath, of an alkaline metal silver cyanide such as KAg(CN).sub.2. The silver cyanide reacts with the chalcogenide layer to form Ag.sub.2 Se which serves as the source for silver migration. A small CN.sup.- concentration, over the stoichiometric concentration for the reaction (1) [Ag(CN).sub.2 ].sup.- .revreaction.CN.sup.- +AgCN, is required for proper bath performance. Too large a CN.sup.- concentration results in Ge-Se layer degradation while a CN.sup.- concentration below stoichiometric for reaction (1) results in the precipitation of the insoluble compound, AgCN. The cyanide concentration produced by the reaction of silver cyanide and the chalcogenide is not sufficient to produce the desired CN.sup.- concentration. The desired cyanide concentration has been obtained according to the teaching of the prior art by adding KCN to the sensitizing bath. While the resists formed by the method described are perfectly adequate for many purposes, it has been found that wafers coated in the same sensitizing bath may exhibit differences in sensitivity.